Interchangeable electrical condition responsive unit



W. H. HOWE INTERCHANGEA ELECTRI CONDITION RE NS U' Filed June 6, 5

Sept. 27, 1949. l

INVENTOR WL'Zf/ec Owe ATTO , Patented Sept. 27, 1949 INTEBCHANGEABLEELECTRICAL CONDI- TION RESPONSIVE UNIT Wilfred H. Howe, Sharon, Man.,assignor to The Foxboro Company, Foxboro, Mass., a eorporstion ofMassachusetts Application June 6, 1945', Serial No. 597,819

22 Claims.

y This invention relates to standardized intery changeable electricalcondition responsive units in which the responsive elements areelectrical resistance type strain gages; and more particularly to unitsof this type for measuring differential pressure, and to such units formaking a plurality of measurements accurately and rapidly at numerousmeasuring points with any standardized measuring instruments without theneed for continuously adjusting such instruments to match the conditionresponsive units.

In accordance with the present invention it is possible to accuratelymeasure the values of the conditions to which a number of conditionsensitive strain gage units are exposed with any standardized measuringinstrument without the necessity for adjusting the measuring instrumentto the individual characteristics of each sensitive unit before eachmeasurement is made, and to provide each of such units in a standardunitary structural form of uniform characteristic such thatva pluralityof such units can be readily and interchangeably connected to themeasuring instrument by multiple-pole switch structures or separableplug connectors without affecting the accuracy of measurement. Althoughin strain gage measurements of the type contemplated in the presentinvention, attempts have been made to compensate for certain of theinherent variables in strain gage elements which prevented suchinterchangeability of such condition sensitive units, up to the presenttime such attempts have not been entirely successful. In order tocompensate for changes in resistance of the strain gage elements causedby changes in their ambient temperature, and also in order to multiplytheir sensitivity, it has been usual to use four substantially identicalstrain gage elements connected in a Wheatstone bridge type circuit. Twoof these elements are arranged so as to be responsve to the strain beingmeasured and are connected in two opposite arms of the bridge circuit,and the other two are arranged in close proximity to the first two so asto be exposed to the same ambient temperature but not subjected to anystrain and are connected in the other two opposite arms of the bridge;as shown,

for example, in the copending application of Albert Allen, Serial No.543,089, which has become abandoned. Such an arrangement eectivelycompensates for changes in bridge unbalance caused by changes in theresistance of the strain gage elements due to changes in their ambienttemperature because the two temperature compensating resistors providean equal and op- 2 posite effect. Furthermore, increased sensitivity isobtained with this arrangement because the two strain-sensitive elementsare in opposite arms of the bridge, thus acting additively to provide adouble unbalance effect for any strain condition. A change in thecondition of balance of the bridge circuit, due to change in resistanceof the two strain sensitive elements caused by change in the strain ofthose elements, may be measured by any suitable bridge balance measuringarrangement, or by a double bridge measuring arrangement such as thatshown in my copending application Serial No. 496,438. In addition toproviding a convenient measuring instrument, this kdouble bridge circuitarrangement also minimizes the effect of circuit resistance changescaused by variation in switch contact/resistance and the like. However,none of these arrangekments permits the provision of completelyinterchangeable sensitive units because of other variations inherent inthe structure of such units. Such variations are due to differences inthe precise dimensions and the modulus of elasticity of the material ofone unit as compared with that of another', by changes in this moduluswith changes in temperature, by dierences in the specific sensitivitybetween units, by differences in the zero balance values, and the like.These variations prevent such arrangements from providing absolutelyuniform measurements from a plurality of such condition sensitive units,or with a single unit from a plurality of measuring instruments, unlessall of the strain gage elements are carefully matched, and matched tothe measuring instrument; or unless the measuring instrument is adjustedto the individual characteristics of each condition sensitive unit afterit is connected to the measuring instrument but before a measurement ofvthat unit is made. In accordance with the present invention suchcomplete interchangeability is accomplished by the provision in thecondition responsive unit of standardizing resistance/units of specialcharacteristics which adjust and compensate for the idiosyncrasies andvariations in characteristics inherent in the electrical and ymechanicalcharacteristics of the individual strain gage elements.

Further, in accordance with the present invention, it is possible tomeasure the pressure in a container, or to measure differentialpressure, by means of such resistance type strain gage responsive units.This is accomplished by measuring the strain in a member, strained byapplication of the pressure to be measured, by

means of an electrical strain gage attached to this member. Such apressure responsive device Vmay be in the form ofthe tubular structureshown in my copending application, Serial No. 549,532, i. e. a tubearound which isspirally wound a wire whose electrical resistance variesin accordance with its strain. This wire is afxed to the surface of thetube in some suitable manner, such, for example, as by being firmlybonded thereto with some cementing substance, or by being wound thereonin a strained condition, as described in my above-mentioned copendingapplication. In such a device the stress induced in the tubular memberdue to the application of a differential pressure between the inside andthe outside thereof causes the tube, and therefore the wire due to itsmechanical attachment to the surface of the tube, to be strained, thuschanging the resistance of the wire. This permits measurement ofdiierential pressure across the tube with any Well-known resistancemeasuring means by measuring the change of resistance of the wire.Further, in accordance with the present invention, multiple pressuremeasurements may be made with any standardized measuring instrument anda plurality of such pressure sensitive units.

Accordingly, therefore, it is an object of this invention to provideelectrically uniform resistance strain gage type condition responsiveunits provided with standardizing and compensating elements so that suchunits are completely interchangeable, and which permit any standardizedindicating, controlling, and/or recording instrument to operateinterchangeably and uniformly with any such condition sensitive uni-t.It is another object of this invention to provide such units which areresponsive to differential pressures, and which may be constructed tohave a standardized and uniform response characteristie throughout allranges of normal operating temperatures.

These and other apparent objects and advantages of this invention areobtained by the means described in the following specification and maybe more readily understood by reference to the accompanying drawingswherein:

Figure 1 is an exploded view in perspective of a standardizeddifferential pressure responsive unit provided with a separable plugconnector permitting it to be interchanged with other such units, all inaccordance with the present invention;

Figure 2 is a vertical cross section of the pressure responsive unitshown in Figure 1;

Figure 3 is a section taken on line 3-3 of Figure 2; and

Figure 4 is the schematic wiring diagram of a suitable electricalinstrument for determining the response of the pressure sensitive unitshown in Figure 1.

The various features of the invention can be most readily described byreference to Figure 4 of the drawings which shows the wiring diagram ofa differential pressure responsive system embodying the invention. Thissystem comprises an interchangeable pressure responsive unit, generallyindicated at I, and a bridge-balance measuring device, generallyindicated at 3, interconnected by a detachable connector arrangementcomprising a separable connector plug, generally indicated at 5, and a,corresponding receptacle, generally indicated at 1. The interchangeablepressure responsive unit l includes eight resistor units P-|, P-2, T-IT-Z, B, S, M and Z, which are interconnected, as shown in Figure 4, toform a Wheatstone bridge type circuit, generally indicated at 9, havingfour terminals 50. 52. N and l connected by leads 60, 62, 64 and l0,respectively, to the proper pins 20, 22. 24 and 28, respectively, onplug 5, so that when plug l is inserted in receptacle 1 the bridge 9 isexcited through leads B4 and E6 and its condition of balance isdetermined by measurement between leads 60 and 02. Resistors P-I and P-2are resistance type strain gage elements applied in the mannerhereinafter to be described to a mechanical structure adapted to bestrained by application thereto of the pressure to be measured wherebythe strain indicated by the strain gage elements is a measure of thispressure. These two pressure responsive strain gage resistors P-I andP-l are connected in opposite arms of the bridge 9 between the terminals50 and 54, and i2 and 56, respectively. A change in the pressure to bemeasured changes the resistance of resistors P-i and P-2 in the samedirection and, because they are positioned in opposite arms of thebridge 9, this causes the change in the condition of balance of thebridge 9 to be double the amount that would result from the use of onlyone such resistors. Thus this circuit arrangement effectively doublesthe sensitivity of the apparatus.

Resistors T-I and T--2 are temperature compensating resistors which aresubstantially identical in dimensions, material, resistance, andresistance-temperature coefdcient to the pressure responsive resistorsP-I and P2; and they are mounted in such close proximity to resistorsP-i and P-2 as to be exposed tosubstantially the same temperature. Thesetwo temperature compensating resistors T-i and T-2 are connected in theother two opposite arms of the bridge circuit 9 between the terminals 50and 5B and 52 and 54, respectively, so that any unbalance of bridgecircuit 9, caused by change in resistance of resistors P-I and P--2 dueto change in their ambient temperature, is exactly compensated for by anequal change of resistance of the resistors T-I and T-2, which changeaffects the condition of balance of bridge 9 in an equal and oppositemanner.

Because it is impossible in practice to construct four resistors P--|,P-2, T--i and T2 which are exactly identical in electrical andmechanical characteristics, a bridge balancing resistor B is providedconnected in one arm of the bridge 8. This resistor B is of such valueand is connected in such arm of the bridge i, e. g. in the arm of thebridge 9 between its terminals l2 and B6 in series with pressureresponsive resistor P-l as shown in the drawings, so as to bring thebridge circuit 9 into balance at some base pressure value such as acondition of zero pressure applied to the pressure responsive resistorsP-I and P-I. In order that this bridge balancing resistor B willmaintain the bridge 9 in balance at a condition of zero pressureregardless of the ambient temperature of the pressure responsive unit i,this resistor B is made of material and in a manner substantiallyidentical to that of resistors P-I, P-I, T-I and T-2, so that it hassubstantially the same thermal and electrical resistancecharacteristics.

Changes in the temperature of the pressure responsive unit i may a'ectthe modulus of elasticity of the mechanical structures on which thepressure responsive resistors P-I and P-I are mounted. Such changesaiiect the uniformity of the response of the unit l. Since the modulusof elasticity of most materials tend to decreasev perature, it ismounted in close proximity to the remainder of the resistors in thebridge circuit 9 so that its temperature corresponds substantially withthe temperature of t'he structures on which these resistors are mounted,and it is connected in one of the leads which supplies the excitation tothe bridge circuit 9, e. g. in lead 84 connected to terminal 54 as shownin Figure 4. With this arrangement, any increase in temperature oi thepressure responsive unit I causes a corresponding increase in theresistance of this modulus compensation resistor M and thus tends todecrease the excitation voltage applied to the bridge circuit 9. Thisaction renders the bridge circuit less sensitive at higher ambienttemperatures so that a given pressure change at a higher temperatureproduces the same bridge unbalance voltage that the same pressure changewould produce at a lower temperature, By careful selection of thetemperature-resistance coeflicient and the resistance value of resistorM, this decrease in sensitivity, l. e. decrease in bridge excitation,can be made to exactly oiset the increase in sensitivity of the pressureresponsive units, i. e. resistors P-I and P-2, caused by the decrease inthe modulus of elasticity of their mechanical mounting structures due toincrease in their ambient temperature. Changes in the sensitivity, i. e.the change in the bridge unbalance cond-ition caused by a given changein the pressure applied to pressure responsive resistors P-I and P--2,of individual pressure responsive units may result from such things assmall variations in the dimensions of the mechanical structures on whichthe pressure responsive strain gage elements P-I and P-2 are mounted,small diierences in the sensitivity of the individual strain gageelements themselves, small diierences in the overall impedance of theindividual bridge circuits due to diierence in temperature compensatingresistance values, and the like. In order to eliminate such variationsin sensitivity between various pressure responsive units, a resistor Sis provided in each unit to set the overall sensitivity of the unit tosome predetermined value which is a standard for all units. Thisresistor S is connected in series in one of the leads that suppliesexcitation to the bridge circuit 9, e. g. in lead 64 connected toterminal 54 in series with resistor M, and its value of resistance is sochosen that the bridge excitation is just suilicient to provide thedesired sensitivity. The specified standard sensitivity for allinterchangeable pressure units is set at a suliciently low value sothatit is possible tok obtain this specified sensitivity on all units bysuitable choice of the values of sensitivity control resistors S.

The resistor Z, which is connected across the excitation leads to thebridge, e. g. between leads 84 and 66 connected to terminals 54 and 56,respectively, of the bridge circuit 9, is provided as an impedancematching resistor to maintain the apparent resistance between the bridgeexcitation terminals at some value which is standard for allinterchangeable pressure responsive units. The value of this resistor Zis so chosen that it reduces the input resistance across the excitationterminals of such interchangeable pressure responsive units to somesuitable standard value.

Although the bridge circuit' of the pressure responsive unit I, justdescribed, may be excited in any suitable manner either from an A. C. ora D. C. source, and although its condition of balance may be determinedby any suitable means.

it has been found preferable to excite such a` bridge from an A. C.source of power and to measure its condition of balance by means of abalancing bridge connected in a double bridge circuit as described in mycopending application Serial No. 496,438. In a measuring system of thetype contemplated by the present invention, using a plurality ofinterchangeable condition sensitive elements of the type hereindescribed, connected to the measuring instrument by separable plugconnectors wherein variable contact resistance eilects may beencountered, the double bridge" type system has particular advantages aspointed out in the above-mentioned copending application. Referring toFigure 4, a simplied doublebridge measuring instrument is generallyindicated at 3. This comprises a suitable source of A. C. potential 10,such. for example, as a vacuum tube oscillator, operating at somesuitable ire-A quency, such, for example, as 1000 cycles, which, througha transformer 12 having a single primary winding 14, connected to the A.C. power source 10, and two substantially identical secondary windings16 and 18, excites the bridge circuit 9 of pressure responsive unit Iand a balancing bridge circuit, generally indicated at II, which isadapted to be unbalanced in any predetermined amount by operation of aslide wire 82. The secondary winding 16 is connected to the excitationterminals II4, II6 of the balancing bridge II, and the other secondarywinding 18 of transformer 12 is connected through leads 94 and 9S ofcable 48, plug receptacle 1, pins 24 and 2B in plug 5, to leads 64 and66, respectively, to excite the bridge circuit 9 of the pressureresponsive unit I. A matching resistor 19 is connected in lead 96 topermit the excitation voltages of the two bridges 9 and I I to be set atthe proper ratio determined by the calibration of the slidewire 82. Thebalance detecting terminals 50 and 52 of the bridge circuit 9 in theresponsive unit I are connected through leads 60 and 62, pins 20 and 22in plug 5, receptacle 1 and leads 90 and 92 of cable 48 to the balanceterminals III) and II2 ol balancing bridge II through a null indicatordevice 84. With this arrangement, any unbalance in bridge 9 of pressureresponsive unit I produced by changes in the pressure applied to thisunit, is measured by adjusting the balancing bridge II to an equal andopposite condition of unbalance so that the indicator 84 indicates anull condition. Under such circumstances, the amount of adjustment ofthe slidewire 82 is a direct measure of the pressure applied to thepressure responsive unit I. In accordance with the present inventionlany number of standardized pressure responsive units of the type hereindescribed may be interchangeably connected to the measuring instrument 3through the separable plug receptacle 1 and a direct reading of thepressure to which they are exposed may be made by reading of theposition of slidewire 82, and this may be accomplished without the needfor any preliminary adjustments of the measuring instrument 3 to adaptit to the 7 individual characteristics of the individual pressureresponsive umts so connected.

Referring now to Figures 1, 2 and 3 of the drawings, there is showntherein a standardized interchangeable differential pressurev responsiveunit which incorporates the features of the invention described above inconnection with the wiring diagram shown in Figure 4. This arrangementillustrates one form in which a standardized and interchangeablecondition sensitive element may be made in a simple and compact unitarystructure which may be quickly and easily associated or disassociatedboth with the condition being measured and the measuring instrument usedfor indicating, recording and/or controlling the value of thiscondition. This diiferential pressure responsive unit is provided with ahollow cylindrical base member, generally indicated at 2, externallythreaded at one end I and at the other end 6, and provided with ahexagonal shoulder portion 8. The threaded end 4 is adapted to bescrewed into a suitably tapped hole (not shown) in-any container inwhich the pressure is to be measured, as, for example, into the sparkplug hole of an internal combustion engine cylinder block wherein it maybe firmly seated by application of a wrench to the hexagonal shoulderportion B. A cylindrical cavity is provided in the base 2 extendinglongitudinally therein from the end 4 to form a hollow pressure chamber,generally indicated at I0, which, when the pressure unit is attached toa container in the manner abovementioned, communicates with the interiorof such container. Three solid rod-like members or posts I2, I4 and I6,and a hollow or tubular member I8, are fastened to the end 6 of the base2 in some suitable manner as by being threaded into holes 3U so as to beparallel to each other and project longitudinally therefrom. The freeend of the tubular member I8 is closed, as shown in Figure 2, so that iteffectively becomes an extension of the pressure chamber I0. Twostrainsensitive resistance elements P-I and P--2 are wrapped around thetubular member I8, as by spirally winding around tubular member I8 twocontinuous laments of a material whose electrical resistance varies inaccordance with its strain and firmly aiiixing these filaments to theouter surface of this tubular member I8, for example, in the mannerdescribed in my copending application Serial No. 549,532. With thisarrangement, when pressure is applied to the inrior of pressure chamberI0, and thus to the interior of the tubular member I8, i. e., when adiiferential pressure is applied across the inside and the outside wallsof the tubular member I8, a stress is developed in its walls causingthem to be strained. This strains the wires in resistors P-I and P-2,causing them to change their resistance. This change in resistance is ameasure of the difierential pressure applied to the tubular member I2.Although this change in resistance of the strain sensitive resistanceelements P-I and P--2 can be measured in any suitable manner, this ispreferably accomplished in a manner hereinbefore described.

Two other resistance elements T-I and T2, substantially identical withpressure-sensitive resistances P-I and P-2, are wrapped around one ofthe posts, e. g. around post Il as shown in the drawings. These twcresistors T-I and T--2, be-

assasoc sistors to compensate for changes in the reshtanee I u ingsubstantially identical to pressure sensitive resistors P-I and P-2, andbeing exposed to substantially the same temperature as these resistors,operate as temperature-compensating reof strain sensitive ristors P-I.and P--2 caused by changes in their temperature, in the .mannerdescribed above in connection with Figure 4.

Two otherresistors B and B are wrapped around one of the other posts, e.g. post I2 as shown inthe drawings, the resistor B to act as abridgebalance setting resistor, and the resistor S to act as a bridgesensitivity determining resistor for the purposes described above.

The two other resistors M and Z are wrapped around the remaining postI6. The resistor M serves to compensate the changes in thecharacteristics of the pressure sensitive unit I caused by changes inthe modulus of elasticity of the tubular member I8. The resistor Zserves to adjust the overall look-in impedance of the pressureresponsive unit bridge circuit l to some predetermined standardizedvalue, as above mentioned. All of these resistors P-I, P-2, T-i, T-2, B,S, M and Z are connected together as shown in Figure 4 to operate inamanner abovedescribed, and the four leads 60, 02, il and II of thisbridge circuit 9 are soldered to the proper terminal pins 20, 22, 24 and26, respectively, o! the connection plug member 5. This plug member I isattached to the base 2 to form a unitary structure therewith by acylindrical sleeve Il, one end of which is threaded to the upper end iof the base 2. The other end of sleeve 34 is provided with an internalperipheral groove 2B to receive the plug assembly 5 where it is held inplace by a split expansion ring 38. The unitary pressure responsivedevice thus formed is adapted to be connected to a measuring instrumentI by lplugging a receptacle I attached to a connection cable 48 on tothe plug assembly l. The plug end of the sleeve 34 is also threaded onthe outside to coact with a threaded clamping collar 42 adapted to screwdown on a shoulder 46 on plug receptacle 1 to hold the plug receptacle 1firmly in contact with plug member 5 to connect the pressure responsiveunit I to the measuring instrument 3.

The following are an example of actual values which have been found tobe quite satisfactory lin producing standardized interchangeablepressure responsive units of the type herein described. Resistors P-I,P--2, T-I, and T-Z were wound of approximately ilve inches of .001 inch0. D. constantan wire and each had a resistance of approximately 120ohms; resistor B was wound ot .005 inch O. D. constantan wire and had aristance of less than l ohm; resistor S was wound of .003 inch O. D.constantan wire and had a resistance in the neighborhood of 10 ohms;resistor M was wound of .003 inch O. D. nickel wire and had a resistanceof about 5 to l0 ohms; and resistor Z was wound of constantan wire ofany convenient diameter and had a resistance of approximately 1000 ohms.A pressure responsive unit having a circuit with values approximatingthose above stated had a look-in impedance at its excitation terminals64 and 66 of approximately 120 ohms. Its pressure sensitivity would bedetermined primarily by the material and dimensions of the tubularmember Il.

Although in the pressure responsive unit embodying the invention hereindescribed the various standardizing and compensating resistors havebeen-shown mounted on four projections on the base 2. it is obvious tothose skilled in the art that these resistors might be mounted on anynumber of projections and in any manner desired, providing they are al1exposed to substantially the same operating temperatures and had theother necessary characteristics described above.

Since many possible embodiments might be made of the above invention,and since many changes might be made in the embodiments above set forth,all without departing from the scope of the invention. it is to beunderstood that all matter hereinbefore set forth or shown in theaccompanying drawings is to be interpreted as illustrative only and notin a limiting sense.

I claim: f

l. A strain gage type pressure responsive unit comprising, incombination, a hollow cylindrical pressure chamber having an opening inone side thereof, mounting means for attaching said pressure chamber toa container in which pressure is to be measured so that the interior ofsaid pressure chamber communicates with the interior of said containerthrough said opening, a solid cylindrical member of d'mensions andmaterial substantially equivalent to those of said pressure chambermounted in close proximity thereto, four substantially identicalresistance strain gage elements, two of said elements being mechanicallyassociated with, the outer surface of said pressure chamber to bestrained with the walls of said chamber due to the stress producedtherein caused by change in the differential pressure between the insideand the outside of said chamber to act as pressure responsive elements,the other two of said elements being associated with the outer surfaceof said solid member so as to be subjected to substantially the sametemperature as said ilrst two elements to act as temperaturecompensating elements, means connecting said four elements in aWheatstone bridge type. circuit, said two pressure responsive elementsbeing connected in two opposite arms of said bridge circuit and said twotemperature compensating elements being connected in the other twoopposite arms of said bridge circuit, and means for connecting saidbridge circuit to bridge balance responsive means whereby the pressurein said container may be determined by determination of the state ofbalance of said bridge circuit.

v2. A strain gage type pressure responsive unit for measuring thepressure in a container, comprising, in combination, a base havingmounting means for securing the same to close a hole in said container,a tubular member having one closed end mounted on said base so that thehole in its other end communicates with the hole in said container, asolid cylindrical member of external dimensions and materialsubstantially equivalent to those of the tubular member mounted on saidbase in close proximity to said tubular member, four continuousfilaments of material whose electrical resistance` varies in accordancewith its strain, two of said filaments being wound spirally around andailixed to the surface of said tubular member to be strained with thewalls thereof due to stress therein caused by change in the differential.pressure between the inside and outside of said tubular member, theother two of said filaments being wound and ailixed in a substantiallyidentical manner to the surface of said solid member, means connectingthe four resistances thus formed in a, Wheatstone bridge type circuit,said ilrst two resistances being 'connected i in two opposite arms ofsaid bridge circuit and said last two resistances being connected in theother two arms of said bridge circuit, and means for vconnecting saidbridge circuit to bridge balance responsive means whereby the pressurein 'said container may be determined by `determination of the state ofbalance of said bridge circuit.

3. A straln-gage-type pressure responsive unit comprising, incombination, a hollow pressure chamber having an opening in one sidethereof, means for associating said pressure chamber with a container inwhich pressure is to be measured so that the interior of said pressurechamber communicates with the interior of said container through saidopening, four substantially identical resistance-type strain-gageelements, two of said elements being ailixed to the outer surface ofsaid pressure chamber to be strained with the walls of said chamber dueto the stress therein produmd by change in the differential pressurebetween the inside and the outside of said chamber to act as pressureresponsive elements, means for exposing the other two of said elementsto substantially the same temperature as said ilrst two elements to actas temperature compensating elements, means connecting said fourelements in a Wheatstone bridge type circuit, said two pressureresponsive elements being connected in two opposite arms of said bridgecircuit and said two temperature compensating elements being connectedin the other two' opposite arms of said bridge circuit, a bridgebalance-setting resistor of temperature-resistance characteristicsubstantially similar to that of said four strain gage resistanceelements mounted in close proximity to said elements and connected insuch arm of said bridge circuit and having such a resistance value as tobalance said circuit at some predetermined pressure value in saidcontainer, and means for connecting said bridge circuit to bridgebalance responsive means whereby the pressure in said i container withrespect 'to said predetermined value may be determined by thedetermination i of the state of balance of said bridge circuit.

4. A strain-gage-type pressure responsive unit comprising, incombination, a hollow pressure chamber having an opening in one sidethereof, means for associating said pressure chamber with a container inwhich pressure is to be measured so that the interior of said pressurechamber two elements to act as temperature compensating v elements,means connecting said four elements in a Wheatstone bridge type circuit,said two pressure responsive elements being connected in two oppositearms of said bridge circuit and said two temperature compensatingelements being connected in the other two opposite arms of saidbridgecircuit, a resistor having a relatively hightemperature-resistance coemcient mounted in said unit in close proximitythereto and exposed to substantially the same temperature as said straingage elements connected in series with one of the excitation terminalsof said bridge circuit and having such a resistance value andtemperature-resistance coefficient as to compensate for changes insensitivity of said pressure responsive unit caused by changes inmodulus of elasticity of the material of said pressure chamber due tochanges in its temperature, and Ameans for conl1 necting said bridgecircuit to bridge balance responsive means whereby the lstate of balanceoi said bridge circuit is uniformly responsive to the pressure in saidcontainer for all operating temperatures.

5.; strain-gage-type pressure responsive unit comprising, incombination, a hollow pressure chamber having an opening in one sidethereof, means ior associating said pressure chamber with a container inwhich pressure is to be measured so that the interior oi said pressurechamber communicates with the interior of said container through saidopening, four substantially identical resistance-type strain-gageelements, two of said elements being affixed to the outer surface ofsaid pressure chamber to be strained with the walls of said chamber dueto the stress therein produced by change in the diiierential pressurebetween the inside and the outside oi said chamber to act as pressureresponsive elements, means for exposing the other two oi said elementsto substantially the same temperature i as said ilrst two elements toact as temperature compensating elements, means connecting said fourelements in a Wheatstone bridge type circuit, said two pressureresponsive elements being connected in two opposite arms of said bridgecircuit and said two temperature compensating elements being connectedin the other two opposite arms of said bridge circuit, a bridgebalancesetting resistor of temperature-resistance characteristicssubstantially similar to that oi saidA four strain gage resistanceelements mounted in Iclose proximity to said elements and connected insuch arm of said bridge circuit and having such a resistance value as tobalance said circuit at some predetermined pressure value in saidcontainer, a resistor having a relatively high temperature-resistancecoemclent mounted in said unit in close proximity thereto and exposed tosubstantially the same temperature as said strain gage elementsconnected in series with one of the excitation terminals of said bridgecircuit and having such a resistance value and temperature resistancecoeiilcient as to compensate for changes in sensitivity of said pressureresponsive unit caused by changes in modulus of elasticity of thematerial of said pressure chamber due to changes in its temperature, andmeans for connecting said bridge circuit to bridge balance responsivemeans whereby the state of balance of said bridge circuit is uniformlyresponsive to the pressure in said container with respect to saidpredeterminedl value for all operating temperatures.

8. A strain-gage-type pressure responsive unit for measuringv thepressu're in a. container, comprising, in combination, a base havingmounting means for securing the same to close a hole in -said container,a tubular member having one closed end mounted on said base so that thehole in its other end communicates with the hole in said container, aplurality of solid cylindrical members of external dimensions andmaterial substantially equivalent to those oi the tubular member mountedon said base in close proximity vto said tubular member, four continuousfilaing wound and ailxed in s substantially identical manner to thesurface oi' one oi said solid members, means connecting the fourresistors thus formed in a Wheatstone bridge type circuit, said ilrsttwo resistors being connected in two opposite arms of said bridgecircuit and said last two resistors being connected in the other twoarms of said bridge` circuit, a bridge balancesetting resistor oftemperature-resistance characteristic substantially similar to that oisaid four strain gage resistors spirally wound on one ci said solidcylindrical members and connected in such arm ci said bridge circuit andhaving such a resistance value as to balance said circuit at somepredetermined pressure value in said container, and means for connectingsaid bridge circuit to bridge balance responsive means whereby thepressure in said container may be determined with respect to saidpredetermined value by determination ot the state of balance of saidbridge circuit.

7. Astrain-gage-type pressure responsive unit for measuring the pressurein a container. comprising, in combination, a base having mounting meansioxsecuring the same to close a hole in said container, a tubular memberhaving one closed end mounted on said base so that the hole in its otherend communicates with the hole in said container, a plurality of solidcylindrical members of external dimensions and material substantiallyequivalent to those of the tubular member mounted on said base in closeproximity to said tubular member, iour continuous illaments of materialwhose electrical resistance varies in accordance with its strain, two ofsaid illaments being wound spirally around and aiiixed to the surface ofsaid tubular member to be strained with the walls thereof due to stresstherein caused by change in the diiierentiai pressure between the insideand outside of said tubular member, the other two of said maments beingwound and ailixed in a substantially identical manner to the surface ofone of said solid members, means connecting the four resistors thusformed in a Wheatstone bridge type circuit, said ilrst two resistorsbeing connected in` two opposite arms of said bridge circuit and saidlast two resistors being connected inthe other two arms oi said bridgecircuit, a resistor having a relatively high temperature-resistancecoeiii cient spirally wound on one oi said solid cylindrical members soas io be exposed to substantially the same temperature as said straingage resistors, connected in series with one of the excitation terminalsof said bridge circuit and having such a resistance value andtemperatureresistance coemcient as to compensate for changes insensitivity of said pressure responsive unit caused by changes inmodulus of elasticity of the material oi said tubular member due tochanges in its temperature, and means for connecting said bridge circuitto bridge balance responsive means whereby the state of balance oi saidbridge circuit is uniformly responsive to the pressure in said containerfor all operating temperatures.

8. A strain-gage-type pressure responsive unit i'or measuring thepressure in a container, comprising, in combination, a base havingmounting means for securing the same to close a hole in said container,a tubular member having one closed end mounted on said base so thatthe-hole in its other end communicates with the hole in said container,a plurality of solid cylindrical members of external dimensions andmaterial subr 13A stantially equivalent to those of the tubular membermounted on said base in close proximity to said tubular member, fourcontinuous filaments of material whose electrical resistance varies inaccordance with its strain, two of said filaments being wound spirallyaround and afiixed to the surface of said tubular member to be strainedwith the walls thereof due to stress therein caused by change in thedifferential pressure be` tween the inside and outside of said tubularmember, the other two of said filaments being wound and affixed in a,substantially identical manner to the surface of one of said solidmembers, means connecting the four resistors thus formed in a Wheatstonebridge type circuit, said first two resistors being connected in twoopposite arms of said bridge circuit and said last two resistors beingconnected in the other two arms oi' said bridge circuit, a bridgebalance-setting resistor of temperature resistance characteristicsubstantially similar to that of said four strain gage resistorsspirally Wound on one of said solid cylindrical members and connected insuch arm of said bridge circuit and having such a resistance value as tobalance said circuit at some predetermined pressure value in saidcontainer, a resistor having a relatively high temperature-resistancecoeilcient spirally wound on one of said solid cylindrical members so asto be exposed to substantially the same temperature, connected in serieswith one of the exci tation terminals of said bridge circuit and hav`ing such a resistance value and temperature re- 'sistance coefficient asto compensate for changes in sensitivity of said pressure responsiveunit caused by changes in modulus of elasticity of the 14 settingresistance elementl` of temperature-resistance coeillcient substantiallyequivalent to that of said strain gage resistors spirally woundon one ofsaid solid cylindrical members and connected in such arm of said bridgecircuit and having sucha value of resistance that said circuitisbalanced at some predetermined base presv sure in said container, animpedance-matching resistance velement spirally wound on one of saidsolid cylindrical members and connected across the excitation terminalsof said bridge circuit and having its resistance value so selected as toprovvide standard predetermined look-in impedance lected as to provide apredetermined bridge unmaterial of said tubular member due to changes inits temperature, and means for connecting said bridge circuit to bridgebalance responsive means whereby the state of balance of said bridgecircuit is uniformly responsive to the pressure in said container withrespect to `said predetermined value for all operaitng temperatures.

9. A standard interchangeable strain-gagetype pressure responsive unitfor'measuring the pressure in a container, comprising, in combination, abase having mounting means for securing the same to close a hole in saidcontainer, a tubular member having one closed end mounted on said baseso that the hole in its other end communicates with the hole in saidcontainer, a plurality of solid cylindrical members of externaldimensions and material substantially equivalent to. those of thetubular member mounted on said ,base in close proximity to said tubularmember, four continuous filaments of material whose electricalresistance varies in accordance with its stain, two of saidlaments beingwound spirally around and affixed to the sur- .face of saidtubularmember to be strained with the walls thereof due to stress thereincaused by change in the differential pressure between the inside andoutside of said tubular member, the other two of said filaments beingwound and affixed in a substantially identical manner to the surface ofone of said solid members, means connecting the four resistors thusformed iny a Wheatstone bridge type circuit with the two oppositecorners forming the excitation terminals and the other two oppositecorners forming the balance-detecting terminals of said bridge circuit,said first two resistors being connected in two opposite arms of saidbridge circuit and said last two resistors being connected in the othertwo arms of said bridge circuit, a bridge-balancebalance for apredetermined pressure valuev in said container, and detachableconnection means for connecting said pressure responsive unit to abridge balance determining device, whereby such a standard bridgebalance device may be responsive to the pressures affecting many suchpressure Vresponsive units with respect to their predetermined basepressures without the need forv adjusting said bridgebalancedevice tothe individual characteristics of each of said pressure responsiveunitsv when connected thereto.

l0. A standardized interchangeable straingage-type pressure responsiveunit, comprising, in

combination, a hollow pressure chamber having an opening in vone sidethereof, mounting means for associating said pressure chamber with acontainer in which pressure is to be measured so that the interior ofsaid pressure chamber communicates with the interior of said containerthrough said opening, four substantially identical resistance-typestrain-gage elements, two of said elements Abeing aiiixed to the outersurface of said pressure chamber to be strained with the walls of saidchamber due to the stress therein produced by change in the differentialpressure between the inside and the outside of said chamber to act aspressure responsive elements, means aiiixing the other two of saidelements to said mounting means to expose them to substantially the sametemperature as said first two elements to act as temperaturecompensating elements, means connecting said four elements in aWheatstone bridge type circuit with two opposite corners forming theexcitation terminals and the other two opposite corners forming thebalance detecting terminals of said bridge circuit, said two pressureresponsive elements being connected in two opposite arms of said bridgecircuit and said two temperature compensating elements being connectedin the other two opposite arms of said bridge circuit,bridge-balance-setting resistance means of temperature-resistancecoeilicient sul stantially equivalent to that of said strain gageelements aflixed to said mounting means and connected in such arm ofsaid bridge circuit and having such value of resistance that saidcircuit is balanced at some predetermined base pressure in saidcontainer, impedance-matching resistance means amxed to said mountingmeans and connected across the excitation terminals of said bridgecircuit and having its resistance value so selected as to providestandard predetermined termining device; whereby such a standard bridgebalance device may be responsive to the pressures affecting many suchpressure responsive units with respect to their predetermined basepressures without the need for adjusting said bridge balance device tothe individual characteristics of each of said pressure responsive unitswhen connected thereto.

11. A standardized interchangeable straingage-type pressure responsiveunit comprising, in combination, a hollow pressure chamber having anopening in one side thereof, mounting means for associating saidpressure chamber with a container in which pressure is to be measured sothat the interior of said pressure chamber communicates with theinterior of said container through said opening, four substantiallyidentical resistance-type strain-gage elements, two of said elementsbeing mechanically associated with the outer surface of said pressurechamber to be strained with the walls of said chamber due to the stresstherein produced by change in the dii'- ferential pressure between theinside and the outside of said chamber to act as pressure responsiveelements, means aiilxing the other two of said elements to said mountingmeans to expose them to substantially the same temperature as said firsttwo elements to act as temperature compensating elements, meansconnecting said four elements in a Wheatstone bridge type circuit withtwo opposite corners forming the excitation terminals and the other twoopposite corners forming the balance detecting terminals of said bridgecircuit, said two pressure responsive elements being connected in twoopposite arms of said bridge circuit and said two temperaturecompensating elements being connected in the other two opposite arms ofsaid bridge circuit, bridge-balancesetting resistance means oftemperature-resistl ance coefilcient substantially equivalent to that ofsaid strain gage elements aillxed to said mounting means and connectedin such arm of said bridge circuit and having such value of resistancethat said circuit is balanced at some predetermined base pressure insaid container, impedance-matching resistance means connected across theexcitation terminals of said bridge circuit and having its' resistancevalue so selected as to provide standard predetermined look-in impedanceto said bridge circuit from a source of excitation,sensitivity-determining resistance means formed of material having aresistancetemperature coeilicient of substantially zero value connectedin series with one of the excitation terminals of said bridge circuitbetween it and said impedance-matching resistance means and having avalue of resistance so selected as to provide a predetermined bridgeunbalance for a predetermined pressure value in said container,resistance means having a. relatively high temperatureresistancecoeiilcient aflixed to said mounting means to be exposed tosubstantially the same 16 temperature as said strain gase elementsconnected in series between said sensitivity-determining means and oneof the excitation terminals of said bridge and having a resistance valueand temperature-resistance coemcient selected so as to compensate forchanges in sensitivity of said pressure responsive unit caused bychanges in modulus of elasticity of the material or said pressurechamber due-to changes in its temperature, and detachable connectionmeans for connecting said interchangeable pressure responsive unit to abridge balance determining device whereby such a standard bridge balancedevice, may be responsive to the pressures affecting many such pressureres'ponsive units uniformly throughout all operating temperatures withrespect to their predetermined base pressures without the need foradjusting said bridge balance device to the individ ual characteristicsof each of such pressure responsive units when connected thereto.

12. A standard interchangeable .strain-gagetype pressure responsive unitfor measuring the pressure in a container, comprising, in combination, abase having mounting means for securing the same to close a hole in saidcontainer, a tubular member having one closed end mounted on said baseso that the hole in its other end communicates with the hole in saidcontainer, a plurality of solid cylindrical members of externaldimensions and material substantially equivalent to those of the tubularmember mounted on said base in close proximity to said tubular member,four continuous filaments of material whose electrical resistance variesin accordance with its strain, two of said iilaments being woundspirally around and afilxed to the surface of said tubular member to bestrained with the walls thereof due to stress therein caused by changein the differential pressure between the inside and outside of saidtubular member, the other two of said iilaments being wound in asubstantially identical manner on the surface of one of said solidmembers, means connecting the four resistors thus formed in a Wheatstonebridge type circuit with the two opposite corners forming the excitationterminals and the other two opposite corners forming thebalance-detecting terminals of said bridge circuit. said nrst tworesistors being connected in two opposite arms of said bridge circuitand said last two resistors being connected in the other two arms ofsaid bridge circuit, a bridgebalance-setting resistor oftemperature-resistance coefiicient substantially equivalent to that ofsaid strain gage resistors spirally wound on one of said solidcylindrical members and connected in such arm of said bridge circuit andhaving such a value of resistance that said circuit is balanced at somepredetermined base pressure in said container, an impedance-matchingresistor made of material having a resistancetemperature coeiilcient ofsubstantially aero value spirally wound on one of said solid cylindricalmembers and connected across the excitation terminals of said bridgecircuit and having its resistance value so selected as to providestandard predetermined look-in impedance to said bridge circuit from asource of excitation, a sensitivity-determining resistor made ofmaterial having a resistance-temperature coefiicient of substantiallyzero value spirally wound on one of said solid cylindrical members andconnected in series with one of the excitation terminals of said bridgecircuit between it and said impedance-matching resistor and having avalue 17 ofsoselectedastoprovideapredetarmined bridge unbalance for apredetermined pressure value in said container, a resistor having arelatively high temperature-resistance coemcient spirally wound on oneor said solid cylindrical members so as to be exposed to substantiallythe same temperature as said strain gage resistors connected in serieswith said sensitivitydetermining resistor and one of the excitationterminals oi' said bridge and having a resistance value andtemperature-resistance coemcient selected so as to compensate i'orchanges in sensitivity ci' said pressure responsive unit caused bychanges in modulus of elasticity of the material of said tabular memberdue to changes in its ftemperature, and detachable connection means forconnecting said interchangeable pressure responsive unit to a bridgebalance determining device, whereby such a-standard bridge balanceldevice may be responsive to the pressures ail'ecting many such pressureresponsive units uniformly throughout all operating temperatures withrespect to their predetermined base pressures without the need i'oradjusting said bridge balance device to the individual characteristicso! each of such pressure responsive units when y its closed endprojecting perpendicularly from said base outwardly from said container,a plurality oi rods of external dimensions and material substantiallyequivalent to those of said tube mounted on, said base in closeproximity thereto and projecting perpendicularly therefrom outwardlyfrom said container, fourpieces of Constantan wire of substantiallyequal length and cross-section, two oi said wires being wound spirallyaround and ailixed yto the surface of said tube to be strained with thewalls thereof due to stress therein caused by change in differentialpressure between the inside and outside of said tube, the other two ofsaid wires being wound in a substantially identical manner on thesurface of one of said rods, the four resistors thus formed beingconnected in a Wheatstone bridge type circuit with the two oppositeterminals forming the excitation terminals and the other two oppositeterminals forming the balance-detecting terminals of said bridgecircuit, said rst two resistors being connected in two opposite arms ofsaid bridge circuit and said last two resistors being connected in theother two arms of said bridge circuit, a bridge-balance-setting resistorof Constantan wire spirally wound on another of said rods and connectedinsuch arm of said bridge circuit and having such a value of resistancethat said circuit is balanced at some predetermined base pressure insaid container, an impedancematching resistor of Constantan wirespirally wound on one of said rods and connected across they excitationterminals' of said bridge circuit and having its resistance value soselected as to provide standard predetermined look-in impedance to saidbridge circuit from a source of excitation, a sensitivity-determiningresistor of Constantan wire spirally wound on one oi' said rods andconnected in series with one of the exf citation terminals oi' saidVbridge circuit between 18 it and said impedance-matching resistor andhaving a value of resistance so selected as to provide a predeterminedbridge unbalance for a predetermined pressure value in said container, aresistor of nickel wire spirally wound on one of said rods so as to beexposed to substantially the same temperature as said strain gageresistors connected in series with said sensitivity-determining resistorand one of the excitation terminals of said bridge and having aresistance value selected so as to compensate for changes insensitivityof said pressure responsive unit caused by changes in modulus ofelasticity of the material of said tube due-to changes in itstemperature, and detachable connection means for connecting saidinterchangeable pressure responsive unit to a bridge balance determiningdevice, whereby such a standard bridge balance device may be responsiveto the pressures affecting many such pressure responsive units uniformlythroughout al1 operating temperatures with respect to theirpredetermined base pressures without the need for adjusting said bridgebalance device to the individual characteristics of each of suchpressure responsive units when connected thereto.

14. A standardized interchangeable straingage-type condition sensitiveunit, comprising, in combination, a member adapted to be strained 'inaccordance with the changes in the value of a condition to be measured,'mounting means for associating said member with said condition, foursubstantially identical resistance type strain-gage elements, two ofsaid elements being aillxed to the surface of said member to be strainedtherewith, means aixing the other two of said elements to said mountingmeans to expose them to substantially the same temperature as said firsttwo elements to act as temperature compensating elements, meansconnecting said four elements in a Wheatstone bridge type circuit withtwo opposite corners forming the excitation terminals and the other twoopposite corners forming the balance-detecting terminals of said bridgecircuit, said two condition responsive elements being connected in twoopposite arms of said bridge circuit and saidtwo temperaturecompensating elements being connected in the other two opposite arms ofsaid bridge circuit, bridge-balance-setting resistance -means oftemperature-resistance coeilicient .substantially equivalent to that ofsaid strain gage elements affixed to said mounting means and connectedin such arm vof said bridge circuit andl having such value of resistancethat said circuit is balanced at some predetermined base value of saidcondition, impedance-matching resistance means ailixed to said mountingmeans and connected across the excitation terminals of said bridgecircuit and having its resistance value so selected as to providestandard predetermined look-in impedance to said bridge circuit from asource of excitation, sensitivity-determining resistance means formed ofmaterial having a resistance-temperature coeflicient of substantiallyzero value alxed to said mounting means and connected in series with oneof the excitation terminals of said bridge circuit between it and saidimpedance-matching resistance means and having a value of resistance soselected as to provide a predetermined bridge unbalance for apredetermined change in the value of said condition, and' detachableconnection means for connecting said condition sensitive unit to abridge balance determining device, whereby such a standard bridgebalance device may be re- 19 sponsive to the changes in the conditionsaffecting many such condition sensitive units with respect to theirpredetermined base values without the need for adjusting said bridgebalance device to the individual characteristics of each of saidcondition sensitive units when connected thereto.

15. A standardized interchangeable straingage-type condition sensitiveunit comprising, in combination, a member adapted to be strained inaccordance with changes in the value of a condition to be measured,mounting means for associating said member with said condition, four'substantially identical resistance-type straingage elements, two ofsaid elements being mechanically associated with. the surface of saidmember to be strained therewith, means aiiixing the' other two of saidelements to said mounting means to expose them to substantially the sametemperature as said iirst two elements to -act as temperaturecompensating elements, means connecting said four elements in aWheatstone bridge type circuit with two opposite -corners forming theexcitation terminals and the other two opposite corners forming thebalance-detecting terminals of said bridge circuit, said two velementsassociated with said condition responsive member being connected in twoopposite arms of said bridge circuit and said two temperaturecompensating elements being connected in the other two opposite arms ofsaid bridge cirbridge balance device to the individual chamcuit,bridge-balance-setting resistance means of temperature-resistancecoeiiicient substantially equivalent to that of said strain gageelements ai'iixed to said mounting means and connected in such arm ofsaid bridge circuit and having such value of resistance that saidcircuit is balanced at some predetermined base value of said condition,impedance-matching resistance means afnxed to said mouting means andconnected across the excitation terminals of said bridge circuit andhaving its resistance value so selected as to provide standardpredetermined lookin impedance to said bridge circuit from a source ofexcitation, sensitivity-determining resistance means formed of materialhaving a resistancetemperature coeilicient of substantially zero valueaffixed to said mounting means and connected in series with one of theexcitation terminals of said bridge circuit between it and saidimpedance-matching resistance means and having a value of resistance soselected as to provide a predetermined bridge unbalance for apredetermined change in the value of said condition, resistance meanshaving a relatively high temperature-resistance coefcient aiiixed tosaid mounting means to be exposed to substantially the same temperatureas said strain gage elements connected in series with saidsensitivitydetermining means and one of the excitation terminals of saidbridge and having a resistance value and temperature-resistancecoefficient selected so as to compensate for changes in sensitivity oi'said condition sensitive unit caused by changes in modulus of elasticityof the material of said member due to changes in its temperature, anddetachable connection means ior connecting said interchangeablecondition sensitive unit to a bridge balance determining device, wherebysuch a standard bridge balance device may be responsive to the changesin the conditions affecting many such condition sensitive unitsuniformly throughout all operating temperatures with respect to theirpredetermined base values without the need for adjusting said terlsticsoi' each of such condition sensitive umts connected thereto.

16. A standard interchangeable strain-gagetype condition sensitive unitfor measuring the value of a condition,v comprising, in combination, asupport, a ilexible member attached to said support and adapted to bestrained in accordance with the changes in' the value oi a condition tobe measured, said support having mounting means for positioning the sameto associate said flexible member with the condition to be measured, aplurality of relatively inflexible members of material substantiallythermally equivalent to that of said flexible member mounted on saidsupport in close proximity to said flexible member, four continuousiilaments of material whose electrical resistance varies in accordancewith its strain, two of said laments being arranged on and aiilxed tothe surface of said flexible member with their greatest length in thedirection of maximum strain to be strained therewith due to stresstherein caused by change in the value of said condition, the other twoof said ilisments being arranged in a substantially identical manner onvthe surface of one of said inflexible members, means connecting thefour resistors thus formed in a Wheatstone bridge type circuit with thetwo opposite corners forming the excitation terminals and the other twoopposite corners forming the balance-detecting terminals of said bridgecircuit, said first two resistors being connected in two opposite armsof said bridge circuit and said last two resistors being connected inthe other two arms of said bridge circuit, a bridge-balance-settingresistor of temperatureresistance coeilicient substantially equivalentto that of said strain sensitive resistors aillxed to one of saidiniiexible members and connected in such arm of said bridge circuit andhaving such a value of resistance that said circuit is balanced at somepredetermined base value of said condition, an impedance-matchingresistor aiiixed to one of said iniiexible members and connected acrossthe excitation terminals of said bride circuit and having its resistancevalue so selected as to provide standard predetermined look-in impedanceto said bridge circuit from a source of excitation, asensitivity-determining resistor made of material having aresistance-temperature coefiicient of substantially zero value aillxedto one of said inflexible members and connected in series with one ofthe excitation terminals of said bridge circuit between it and saidimpedancematching resistor and having a value of resistance so selectedas to provide a predetermined bridge unbalance for a predeterminedchange in the value of said condition, a resistor having a relativelyhigh temperature-resistance coemcient ailxed to one of said inflexiblemembers so as to be exposed to substantially the same temperature assaid strain sensitive resistors connected in series with saidsensitivity-determining resistor and one of the excitation terminals ofsaid bridge and having a resistance value and temperature-resistancecoeiilcient selected so as to compensate for changes in sensitivity ofsaid condition sensitive unit caused by changes in modulus of elasticityof the material of said ilexible member due to changes in itstemperature, and detachable connection means for connecting saidinterchangeable condition sensitive unit to a bridge balance determiningdevice, whereby such a standard bridge balance device may be responsiveto the changes in the conditions affectto the individual characteristicsof each of such condition sensitive umts when connected thereto.

17. A standardized interchangeable straingage-type pressure responsiveunit, comprising, in combination, a hollow `pressure chamber having anopening in one side thereof, mounting means for attaching said pressurechamber to a container in which pressureis to be measured so that theinterior of said pressure chamber communicates with the interior of saidcontainer through said opening, a solid member of dimensions andmaterial substantially equivalent to those of said pressure chambersecured to said mounting means in close proximity to said pressurechamber, four substantially identical resistance-type. strain gageelements, two of said elements being aixed to the outer surface of saidpressure chamber to be strained by stress produced in the walls of saidchamber due to change in the differential pressure between the insideand outside of said chamber, to act as pressure responsive elements, theother two of said elements being aiiixed to the outer surface yconnected in opposite arms of said bridge circuit and said twotemperature compensating elements being connected in the other twoopposite armsv of said bridge circuit, a balance-setting resistorconnected between one of said pressure responsive elements and one ofsaid temperature compensating elements, a sensitivity determiningresistor, a modulus-change compensating resistor, an impedance matchingresistor, circuit means connecting these three last-named resistors inseries between one end of said balance-setting resistor and the oppositeterminal of said bridge, detachable connection means for connecting asource of energizing potential across said impedance matching resistorand for connecting the opposite terminals of said bridge circuit to abridge balance responsive device.

18. An electrical resistance strain-gage-type condition responsive unit,comprising, in combination, a member adapted to be strained inaccordance with changes in the value of a condition, a Wheatstone bridgetype circuit, an electrical resistance type strain gage element ailixedto said member to be strained therewith and connected in one arm of saidbridge circuit, first connection means through which an energizingpotential may be applied across two opposite terf minals of said bridgecircuit, second connection means through which measuring means adaptedto be responsive to the condition of balance of said bridge circuit maybe connected to the other two terminals of said circuit for determiningthe value of said condition in terms of the unbalance of said circuit, atemperature sensitive resistor mounted in close thermal proximity tosaid member and connected in said iirst connection means in series withone of the energization terminals of said bridge circuit to reduce theenergization thereof with increase in the temperature of said member,whereby the response of said unit is maintained v substantially constantin spite of changes in sensitivity of said element caused by changes inthe 19. An electrical resistance strain-gage-type4 condition responsiveunit, comprising, in combination, a member adapted to be strained inaccordance with changes in the value of a condition, an

electrical resistance-type strain gage element' mechanically associatedwith said member to be strained therewith, a measuring circuit, meansconnecting said element in said measuring cir,- f

cuit to change the distribution of electrical potential therein inresponse to change in the value oi the resistance of said element as ameasure of the value of said condition, means for connecting to saidmeasuring circuit a measuring instrument adapted to be responsive to thepotential distribution of said measuring' circuit for determining thevalue of said condition, circuit means for applying an energizingpotential to said measuring circuit, a temperature sensitive resistormounted so as to be exposed to the same operating temperature as saidmember, means interconnecting said resistor in said energizing circuitmeans to reduce the energization 'of said measuring circuit as thetemperature of said member increases, whereby an increase in thesensitivity of said circuit with increase in the temperature of saidmember due to change in the modulus of elasticity of said member iscompensated for by a decrease in the energization of said vmeasuringcircuit.

20. A standardized interchangeable straingage-type condition sensitiveunit, comprising, in combination, a member adapted to be strained inaccordance with the changes in 'the value of a condition to be measured,mounting means for associating said member with said condition, foursubstantially identical resistance-type straingage elements, two of saidelements being aiiixed to the surface of said member to be strainedtherewith, means aillxing the other two of said elements to saidmounting means to expose them to substantially the same temperature assaid rst two elements to actas temperature compensating elements, meansconnecting said four elements in a Wheatstone bridge type circuit withtwo opposite corners forming the excitation terminals and the other twoopposite corners forming the balance-detecting terminals of said bridgecircuit, said two condition responsive elements being connected in twoopposite arms of said bridge circuit and said two temperaturecompensating elements being connected in the other two opposite larms ofsaid bridge circuit, impedance-matching resistance means ailixed to saidmounting means connected across the excitation terminals of said bridgecircuit and having its resistance value so selected as to providestandard predetermined look-in impedance to said bridge circuit from asource of excitation, and detachable connection means for connectingsaid condition sensitive unit to a bridge balance determining device,whereby such a standard bridge balance device may be responsive to thechanges in the conditions affecting many such condition sensitive unitswithout the need for adjusting said bridge balance device to theindividual characteristics of each of said condition sensitive unitswhen connected thereto.

21. A standard interchangeable strain-gagetype pressure responsive unitfor measuring the pressure in a container, comprising, in combination, abase having mounting means for securing the same to close a hole in saidcontainer, a tubular member having one closed end mounted 23 a pluralityof solid cylindrical members of external dimensions and materialsubstantially equivalent to those of the tubular member mounted on saidbase in close proximity to said tubular member, four continuousiilaments of material whose electrical resistance varies in accordancewith its strain, two of said filaments being wound spirally around andamxed to the surface of said tubular member to be strained with thewalls thereof due to stress therein caused by change in the differentialpressure between the inside and outside of said tubular member, theother two oi said nlaments being wound and aiiixed in a substantiallyidentical manner to the surface of one of said solid members, meansconnecting the four resistors thus formed in a Wheatstone bridge typecircuit with the two opposite corners forming the excitation terminalsand the other two opposite corners forming the balance-detectingterminals oi said bridge circuit, said iirst two resistors beingconnected in two opposite arms of said bridgeY circuit and said last tworesistors being connected in the other two arms oi said bridge circuit,an impedance-matching resistance element formed of material having aresistance-temperature co' eilicient of substantially zero valuespirally wound on one of said solid cylindrical members and connectedacross the excitation terminals of said bridge circuit and having itsresistance value so selected as to provide standard predeterminedlook-in impedance to said bridge circuit from a source of excitation,and detachable connection means for connecting said pressure responsiveunit to a bridge balance determining device, whereby such a standardbridge balance device may be responsive to the pressures affecting manysuch pressure responsive units without the need for adjusting saidbridge balance device to the individual characteristics of each of saidpressure responsive units when connected thereto.

22. A standardized interchangeable straingage-type condition responsiveunit, comprising, in combination, a member adapted to be strained gageelementmechanically associated with said member to be strainedtherewith, a measuring circuit, means connecting said element in saidmeasuring circuit to change the distribution oi electrical potentialtherein in response to change in the value oi the resistance of saidelement as a measure ot the value of said condition. means forconnecting to said measuring circuit a measuring instrument adapted tobe responsive to the potential distribution of said measuring circuitfor determining the value of said eondition, energizing circuit meansfor applying an energizing potential from a suitable source to saidmeasuring circuit, impedance-matching resistance means connected acrosssaid energizing circuit means and having its resistance value soselected as to provide standard predetermined look-in impedance to saidmeasuring circuit from said energizing potential source, and detachableconnection means for connecting said condition responsive unit to saidpotential distribution measuring instrument, whereby such a standardpotential distribution measuring instrumentmay be responsive to thechanges in the conditions aiecting many such condition responsive unitswithout the need for ladjusting said measuring instrument to theindividual characteristics o! each of said condition responsive unitswhen connected thereto.

WIIFRED H. HOWE.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Date

